The lower center of gravity allows the car with a firmer lowering springs having less weight shift since the car suspension has less room for body roll.
You have two options coilovers and springs and shocks/dampers.
Coilovers:
Adjustable Coilovers and Coilover Spring Kits are designed to give you the look that you desire. Coil over spring kits allow adjustment anywhere from 0"-4" depending upon its application.
Height is adjusted through use of a threaded sleeve that fits over your stock or aftermarket shocks and a dual locking perch mechanism. Allows lowering of car without changing springs.
Coilovers tend to give a harsh ride compared springs and shocks/dampers option.
You can go as low as 120mm with coilovers.
When you lower a mk4 past 40mm,you may start to have problems with your driveshaft hitting your anti roll bar.You can buy uprated anit roll bars to solve this issue,they have bigger hoops to clear the driveshaft,or run underneath(depending which make you buy).
Once you have done that,the next problem would be your wheel/tyre rubbing against the adjusting rings on the coilover(depending on the wheel offset),to cure this you need 3-5mm wheel spacers.I also recommend you buy longer wheel bolts when you fit any spacer.
Lowing Springs and shocks/dampers:
Lowering springs improve your cars handling by lowering its center of gravity and providing a firmer suspension, more precise ride quality for precision handling.
You can just buy lowering springs and use standard shocks/dampers though thay will wear quicker and buying shocks to match springs is recommended.
Lowering springs for your car allows two benefits to you and your car. Lowering springs give your car better looks and handling suspension with an affordable price.
Springs and shock/dampers generally give better ride quality.
You can not change the ride height though and are stuck with whatever drop you go for 30mm 40mm etc.
You can go as low as 60mm with some shocks and springs.
Note:
Be careful as going lower than 40mm on a mk4 can result in the front Anti roll bar rubbing the driveshaft. This can be overcome by getting an upgraded ARB Or adjustable droplinks. See here for more info
http://www.golfgtiforum.co.uk/index.php?topic=224905.0Whichever you option you go for EIBACH come highly recommended on here -
http://www.eibach.co.uk/.
As with anything you get what you pay for so spend as much as you can afford it will be worth it in the long run